Emergency units, such as exit signs and light fixtures, are commonly used in public buildings to indicate and illuminate areas such as stairs and exits in the event of a power failure. These fixtures typically have an emergency backup power system such as a battery that automatically turns on the emergency light when the primary power source is interrupted.
Once an emergency unit is installed, the batteries need to be replaced every 3 to 5 years. Access to the batteries inside the emergency unit is gained by removing the main front cover to the entire emergency unit. Removing the main front cover exposes the person replacing the battery to voltages from several sources, including line voltage and high voltage from the transformer. Emergency unit batteries should therefore only be replaced by qualified technicians to avoid harm, including electrical shock and death, and damage to the equipment.